1. Field of the Invention
The present invention relates to an apparatus for controlling an internal combustion engine equipped with an evaporative emission control system that captures or traps any fuel vapors coming from a fuel tank mainly when the engine is not running and prevents them from escaping into atmosphere.
2. Description of the Prior Art
As is generally known, a typical evaporative emission control system for an internal combustion engine, has a carbon or charcoal canister filled with activated carbon or charcoal for temporarily storing, trapping or adsorbing fuel vapors emitted from a fuel tank, and a purge control valve disposed in a purge line connecting an induction system with the canister. Generally, the action of clearing or removing the trapped fuel vapor from the canister is called "purging". An electronic air-fuel mixture ratio control system for an internal combustion engine equipped with an evaporative emission control system as discussed above, has been disclosed in Japanese Patent Provisional Publication No. 63-55357. In the air-fuel ratio control system disclosed in the Japanese Patent Provisional Publication No. 63-55357, a flow rate of air passing through a throttle valve is detected, a basic value of a desired fuel-injection amount is, first of all, set on the basis of the air flow rate detected. On the other hand, during a so-called closed-loop mode, an engine control unit (ECU) executes engine air/fuel ratio feedback control in response to a voltage signal from an oxygen sensor, to set a final desired fuel injection amount and to maintain the engine air/fuel ratio almost at a constant value (that is, at as close to stoichiometric as possible), regardless of the presence or absence of fuel vapors escaping from the fuel tank.